I have spent this whole weekend troubleshooting this same problem... Many searches did not turn up this thread until I VALIDATED the Product Display page and removed all errors (one of which was commenting out the REQUIRE to tpl_wishpot.php) and then new WHY Internet Exploder exploded (as it frequently does).
It's because the WISHPOT mod is a FORM and you cannot nest a Form within a Form (that is why you need to place it at the bottom of the page or anywhere Outside the Product Display Form).
However, not that I've tried it yet, I think an idea is to strip out the Form Tags from the Wishpot Mod, write a second Submit button to call a Function that will basically rewrite the ACTION of the submit button (via getElementByID or Name) to do the action of either doing the submit for Add to Cart OR FOR Wishpot...
Just an idea. I will try it later, but my brain is fried and my vision is getting blurred... LOL
BTW, you may want to check out my New Zen Cart for some modifications I've done for Attributes... Nothing fancy, but it works and validates using Javascript (server side coming when I get a Round Tuit...) PLUS, it Updates the default image for both Style and Color after adding to the Cart AS WELL AS in the Shopping Cart. Not bad for just learning Zen Cart in the past two months... And not programming in several years!
Click on my PrettySickPuppy.com link below to take a peek.
Cheers!
Mega Moonshine